1. Field of the Invention
The present invention relates to a thermal head, a thermal printer and a thermal printing method. More particularly, the present invention relates to a thermal head, a thermal printer and a thermal printing method in which surface gloss of recording material is prevented from being lowered after the printing operation at the thermal head.
2. Description Related to the Prior Art
In a color direct thermal recording method of an optical fixation type, a color thermosensitive recording material is used, and includes yellow (Y), magenta (M) and cyan (C) coloring layers overlaid one on another. The coloring layers are heated to develop colors, to record a full-color image. Each of the coloring layers includes micro capsules, coupler and binder. The micro capsules have a sub-micron size, and include a diazonium salt compound as precursor of azo dye as a coloring substance. The coupler and the binder quicken the color development of the micro capsules. In the color development, each coloring layer is heated by a thermal head, to change partitions between the micro capsules to the light-transmitting state, so that the coupler is introduced to the micro capsules to develop the color. In the heating for the magenta, the yellow is prevented from being colored. In the heating for the cyan, the magenta is prevented from being colored. For this prevention, the precursor of the coloring substance of each color is decomposed by application of ultraviolet rays, near ultraviolet rays or visible violet rays, so that each upper coloring layer is kept from being further colored while a coloring layer being next underlaid is heated with relative high heat energy.
In FIG. 25, curves of coloring characteristics of the recording material are illustrated in a graph. The curves represent relationships between coloring density of each of the coloring layers and coloring heat energy generated by heating elements while the thermal head is pressed against the recording material. As is understood from the graph, it is necessary in the color direct thermal recording to use the dynamic range of the coloring heat energy without overlapping between three coloring layers disposed to lie in respective depths in the recording material. If it is desired to set the thermosensitivity of the coloring layers nearly equal to that of other printing methods such as thermal wax transfer printing, then it is required to set a range of the coloring heat energy three times as great as a range of the coloring heat energy according to the thermal wax transfer method. However a range of the coloring heat energy for the three colors is actually set 1.5 or less times as great as a range of the coloring heat energy according to the thermal wax transfer method. This is due to limited heat resistance of the recording material.
Ink ribbon used in the thermal wax transfer method as recording material is discarded after the printing operation. It is possible to construct the ink ribbon only in view of high suitability to thermal printing without considering its final appearance after the printing operation. In contrast final appearance of the recording material for the color direct thermal recording is important after the printing operation, because the recording material should become a print as a finished product in a manner similar to an image receiving sheet used in the thermal wax transfer method. Consequently the recording material must have sufficiently high rigidity and heat capacity. In general it is difficult to contact the recording material being rigid and including paper in a state of readily conducting heat. As is known in the art, the color direct thermal recording requires heat control with higher precision than other thermal printing methods. Furthermore, the color direct thermal recording is associated with a heat contacting condition more difficult than that of other thermal printing methods. It follows in the color direct thermal recording that more stable heat contact should be effected than other thermal printing methods.
In the printer of the color direct thermal recording, the thermal head having partial glaze formed locally in a ridge-shape is used to stabilize heat contact between the thermal head and the recording material, for the purpose of strengthen a head touch of the recording material. The heating elements of the thermal head are arranged on the partial glaze to heighten pressure in the contact by pressing the recording material by a platen roller. The thermal head known in the prior art has the heating elements of which the center is positioned at the top of protruded shape of the partial glaze. Disposition of the platen roller, a pressing condition and a material conveying condition are determined in consideration of stabilized contacting condition of the recording material with the thermal head.
Irrespective of states in which the recording material is pressed against the protruding portion of the partial glaze by the platen roller, there is tension applied to the recording material in a system where a pair of conveyor rollers convey the recording material by drawing it from between the thermal head and the platen roller. In a range downstream from the top of the partial glaze in the conveying direction of the recording material, the tension causes a downstream portion of the recording material to come away from the partial glaze. It is likely that the contacting condition between the recording material and the heating elements at the glaze top is influenced by changes in the tension, irregularity in rotation of the platen roller, and changes in pressure. The contacting state becomes unstable, to change coloring density in an unstable manner.
The type of the recording material is a direct recording medium, of which its recording surface directly heated by the thermal head at high temperature becomes a finally image-reproducing surface. Influence of heat application remains on the surface of the obtained print in a conspicuous manner in comparison with thermal printing with the ink ribbon or the like. Among the coloring layers, the coloring heat energy of the highest value is required to color the cyan coloring layer underlying the lowest of the three. If the cyan is colored at its maximum density, the thermal head becomes as hot as 200 degrees centigrade. If the recording material comes away from the thermal head immediately after passage of the heating elements, the pressure to the recording material abruptly comes down despite the state of the high temperature of the surface and the inside of the recording material. Gas is likely to occur inside the recording material to create blisters or bubbles. The surface of the recording material is likely to be roughened. The surface gloss of the recording material will be lowered.
In any of known methods, it is impossible in the color direct thermal recording to discharge heat from the recording material after passing the recording material. In the range downstream from the top of the partial glaze and upstream from a sheet outlet of the printer, the contact between the thermal head and the recording material is unstable. No good gloss on the recording material is obtainable.
There are various smoothing methods as disclosed in JP-A 2-215569, JP-A 2-233281 and U.S. Pat. No. 5,179,391 (corresponding to JP-A 3-21460), in which the recording material provided with minute protrusions and recesses on its surface is smoothed in a post-process to heighten its gloss. However those require an additional device and additional material for the post-process of glossing separately succeeding to the thermal printing process. The post-process requires manual operation, to complicate the operation of the entirety to a somewhat great extent. It is certain that the device for the post-process could be incorporated in a thermal printer. However the printer thus constructed would be excessively large and expensive.
In view of the foregoing problems, an object of the present invention is to provide a thermal head, a thermal printer and a thermal printing method in which surface gloss of recording material is prevented from being lowered after the printing operation at the thermal head.
Another object of the present invention is to provide a thermal head, a thermal printer and a thermal printing method in which surface irregularity is prevented on recording material from occurring after the printing operation at the thermal head.
In order to achieve the above and other objects and advantages of this invention, a thermal printer includes a conveyor for conveying thermosensitive recording material in a predetermined conveying direction. A thermal head applies heat to the recording material being conveyed, to record an image to the recording material. The thermal head incorporates plural heating elements, arranged in an array crosswise to the conveying direction, for generating the heat. The thermal head includes a contact region predetermined for pressing the recording material, a center of the contact region being positioned down stream from a center of the heating elements with reference to the conveying direction.
The contact region includes a heating surface, disposed on an outside of the heating elements, for conducting the heat to the recording material. A cooling surface is disposed downstream adjacent to the heating surface in the conveying direction, for cooling the recording material.
A platen member is disposed opposite to the thermal head, for supporting a back of the recording material pressed by the thermal head.
The contact region further includes a pre-contact surface disposed upstream adjacent to the heating surface in the conveying direction, and the thermal head satisfies a condition of:
LCRL greater than UCRL
where UCRL is a length of the pre-contact surface with reference to the conveying direction, and LCRL is a length of the cooling surface with reference to the conveying direction.
The platen member is disposed upstream offset from the center of the heating elements in the conveying direction.
The thermal head includes a base plate. A partial glaze is disposed to project from the base plate in a ridge shape with smooth convexity, the heating elements being arranged on the partial glaze, the partial glaze pressing the recording material. The heating elements are disposed upstream offset from a center of the partial glaze in the conveying direction.
In another preferred embodiment, the platen member is disposed upstream offset from a center of the partial glaze in the conveying direction.
In still another preferred embodiment, a rise surface is disposed between the heating surface and an upstream distal end of the partial glaze with reference to the conveying direction, and at least partially curved at a first radius of curvature. The cooling surface is flat or curved at a predetermined radius of curvature, the predetermined radius being greater than the first radius.